Controller design for scalar or multivariable systems whose models are unknown or highly complex are frequently based in practice on the use of a highly simplified approximate plant model. In such circumstances it is vital to be able to quantify the degree of uncertainty to be expected from the use of such a model for prediction of closed-loop characteristics. It is shown how frequency-domain design techniques can be simply extended to incorporate information deduced from the observed differences between open-loop plant and the approximate model step response to quantify this uncertainty and, in particular, to guarantee closed-loop stability and tracking of step demands. A modification of this analysis also yields the possibility of bounding the error in prediction of closed-loop transient performance. The approaches are all graphical in nature and are easily implemented in an interactive computer-aided-design mode.